It is very common to want a decision based on @media screen together with a min-width or max-width to select between layouts. Typically, we would then use lengths based on that same width value to size elements within the layout:

Often, there will be several such @media blocks for each responsive element, based on a series of widths where the layout changes.

Not being able to interpolate the length in the media query here is a significant problem, forcing repetition of those important width values throughout a SCSS file, or across a whole set of files for a larger site instead of storing all core data like that in a single file and @importing it elsewhere.

It might be considered a different issue depending on how the parser works, but for what it's worth, I agree that it would be very nice to interpolate an entire media query as well:

@media $ipad {
// ...
}

For me personally this would have a much lower priority, though, because it is far less likely in practice that something like @media screen is going to change during routine development work.

For those curious, this is the reason I only have -webkit in there. Everywhere else it's unprefixed. Opera 2 versions back has the -o prefix, but opera users tend to be pretty good at upgrading. -moz is totally unneeded.

And, of course, this works with media queries as well. Take a look at Breakpoint's usage of a full media query interpolation.

Finally, for those still curious as to whether or not this is in, well, here's the official changelog about it getting in.